The present invention relates to an imaging/recording device, and more particularly to an electronic still camera for recording a still picture image using the principle of a video camera.
Recently, attention has been paid to the so-called electronic still camera in which a still picture image is recorded on a movable recording medium such as a magnetic disc and a magnetic tape on the basis of the principle of a video camera. The video camera has a large recording capacity to continuously record a moving picture image for a long time. On the other hand, the electronic still camera does not have such a large recording capacity, but is small in size and easy to operate.
The structure of the electronic still camera will be explained briefly, with reference to FIG. 1 showing an example of conventional electronic still cameras. In FIG. 1, reference numeral 1 designates an imaging lens usually formed of a plurality of lenses and incorporating an optical stop for controlling the quantity of incident light, to form an optical image of a subject (namely, a body to be imaged) on an image sensor which will be mentioned later, 2 a mechanical or electrical shutter performing the opening and closing action in response to an electric control signal, 3 an image sensor such as an MOS, charge coupled device, 4 a signal processing circuit for processing a picture image a representing signal read out from the image sensor 3 to form a recording signal recorded on a recording medium, 5 a recording medium such as a rotatable magnetic disc, 6 a control circuit for controlling the operations of various parts (that is, various blocks), 7 a power source for supplying electric power to each of the blocks, 8 a shutter button operated by an operator, 9 a driving device including an electric motor (not shown) to drive the movable recording medium 5, 10 a rotational speed detecting circuit for detecting the number of rotations of the recording medium 5 per unit time (namely, the rotational speed of the recording medium 5), and 11 a scanning circuit for electrically scanning the image sensor 3 to generate a series of signals representative of a picture image formed on the image sensor 3.
FIG. 2 shows an outline of the time sequental processing performed by several portions of the conventional electronic still camera shown in FIG. 1, to explain the operation of the camera.
First, the shutter button 8 is depressed at a time T=T.sub.0. Then, a signal such as shown in part (a) of FIG. 2 is supplied to the control circuit 6, and the power source 7 begins to supply electric power of a predetermined voltage to each block. As shown in part (b) of FIG. 2, the supply voltage applied to each block takes the predetermined value which is a lower limit for operating each block, at a time T=T.sub.1. At this time, the image sensor 3 is changed from an operation stopped state (hereinafter referred to as "A-state") to a scanning/reading state (hereinafter referred to as "B-state") as shown in part (d) of FIG. 2, and other blocks also begin to operate. A time interval between the time T.sub.0 and time T.sub.1 is usually less than 0.1 sec. Since the driving device 9 has a rotating mechanism including an electric motor (not shown) for rotating the magnetic disc, the moment of inertia of a rotating portion is large. Accordingly, the rotational speed of the rotating portion increases gradually as shown in part (c) of FIG. 2, and thus the recording medium 5 reaches a rotational speed which makes possible a recording operation for the medium 5, at a time T=T.sub.2. Although a time interval between the time T.sub.0 and time T.sub.2 depends upon the moment of inertia of the rotating portion and the driving force of the motor for rotating the magnetic disc, it is difficult to make the above time interval less than 0.5 sec because of the size of the camera and the capacity and size of a built-in battery serving as the power source 7. A first drawback of the conventional electronic still camera which will be mentioned later, is caused by the fact that it takes time for the driving device 9 to have a constant rotational speed.
The rotational speed detecting circuit 10 detects the rotational speed of the recording medium 5 to deliver a signal indicating that the rotational speed of the recording medium 5 has reached a constant value. The control circuit 6 responds to the above signal to open the shutter 2 for a period between the time T=T.sub.2 and a time T=T.sub.3 as shown in part (e) of FIG. 2. Thus, light from the lens 1 is allowed to be incident upon the image sensor 3, and an optical image of a body to be imaged is formed on the element 3. The optical image is converted through the photoelectric conversion effect of the image sensor 3 into an electric output, which is read out in the order indicated by the scanning circuit 11 to form a series of signals representative of the optical image. The series of signals is processed by the signal processing circuit 4 simultaneously with the above reading operation so as to have a form suitable for recording, and then recorded on the recording medium 5 as shown in part (f) of FIG. 2. That is, the recording operation is started at the time T=T.sub.3, and completed at a time T=T.sub.4.
The above imaging recording processing has a drawback that a time period between the time the shutter button 8 is depressed and the time the shutter 2 is actually opened, is long, that is, more than 0.5 sec for the main reason that the moment of inertia of the driving device 9 is large. Accordingly, it is difficult to release the shutter at a desired time, and it is actually impossible to obtain a still picture image of a moving body viewed in a desired direction. This is the first drawback of the conventional electronic still camera.
A second drawback of the above camera is that the effective exposure time cannot be made long. In more detail, when the shutter button 8 is depressed, the signal processing circuit 4 performs the recording operation for the recording medium 5 only once. Accordingly, no matter how long the shutter is kept open, the effective shutter speed cannot be made longer than 1/30 sec. That is, the image sensor 3 is scanned in such a manner that a TV signal is formed, and therefore the electric charge which is produced in the element 3 by light, is read out at intervals of 1/30 sec which is twice longer than the period of a vertical synchronizing signal, independently of the state of the shutter 2. At the same time as the electric charge is read out, the signal processing circuit 4 supplies a signal corresponding to the electric charge to the recording medium 5. For a single operation of the shutter button 8, the recording operation is performed only once, and therefore only a signal corresponding to one scanning period for the image sensor 3 is recorded on the recording medium 5. No matter how long the shutter 2 is kept open after the above recording operation has been performed, the electric charge accumulated in the element 3 is merely cleared by the scanning operation at intervals of 1/30 sec, since the element 3 is of a destructive readout type. Thus, no matter how long the shutter 2 is kept open, only the electric charge which is accumulated for a period of 1/30 sec, is supplied to the signal processing circuit 4. Accordingly, one picture image recorded on the recording medum 5 has only an exposure time of 1/30 sec at most. This is the second drawback of the conventional electronic still camera.